What is the maximum setting for the dual-element fuse used for branch-circuit protection of a 1-phase, 7.5 hp, 230 volt motor?

The maximum setting for the dual-element fuse protecting a 1-phase, 7.5 hp, 230-volt motor can be determined using the National Electrical Code (NEC) guidelines, which provide specific rules for motor protection.

For a 1-phase motor, the full-load current can be calculated using a formula that takes into account the horsepower and voltage. For a 7.5 hp motor at 230 volts, the full-load current is typically around 35 amps. Under NEC guidelines, motors are allowed to have branch-circuit protection set at 125% of the full-load current for overload protection. When applying this factor to the full-load current of 35 amps, this results in a maximum fuse rating of approximately 43.75 amps.

However, due to rounding and the availability of standard fuse sizes, the next common standard size would be 50 amps, which is lower than 70 amps but does not fully utilize the allowable rating based on the calculations. This option ensures proper protection for motor start-up currents without causing nuisance tripping, moving towards the understanding that a dual-element fuse is also designed to handle the inrush current effectively without immediate failure.

Therefore, while the initial conclusion may suggest considering various options based